There are three general types of printed circuit (PC) boards. A surface mount board utilizes components that may be secured to a surface of the PC board by an adhesive or by a solder paste. Boards with adhesively secured components are usually sent through a wave solder machine to complete the electrical connections. When solder paste is used to secure components to the board, the solder paste is heated, reflowed and cured to both secure the components to the board and complete the electrical connections. The second type of board uses through hole components. As the name implies, these electrical components have leads that extend through holes or openings in the board. The leads are soldered to complete the electrical connections. In a mixed technology board, a combination of surface mount components and through hole components are used and generally manufactured by combining the methods described above.
In each manufacturing method, a soldering operation is required on one surface of the board. The entire soldering process is comprised of three general steps which are normally performed by a single machine. These steps include (i) flux application, (ii) preheating the board, and (iii) soldering. Soldering flux is generally defined as a chemically and physically active formula which promotes wetting of a metal surface by molten solder, by removing the oxide or other surface films from the base metals and the solder. The flux also protects the surfaces from reoxidation during soldering and alters the surface tension of the molten solder and the base metal. A printed circuit board must be cleaned with flux to effectively prepare the board for soldering with a lead based or other metal based solder paste.
In the manufacture of printed circuit boards or other products, it is frequently necessary to apply minute amounts or droplets of liquid materials, including solder flux and solder paste, to a substrate or workpiece. These droplets can be on the order of 0.10 inch diameter and less. Such materials can generally have a viscosity greater than 25,000 centipoise and in the case of solder pastes, for example, may have a viscosity of 300,000 centipoise or above. These liquid and viscous materials, besides solder flux and solder paste, include adhesives, solder mask, grease, oil, encapsulants, potting compounds, inks, and silicones.
Methods of applying minute drops of liquid or viscous material have, for example, relied on syringes or other positive displacement devices. Typically, as discussed in U.S. Pat. No. 5,320,250, syringe dispensers place the syringe tip of the dispenser very close to the substrate. This may be a distance of 0.005 inches for a very small droplet and a distance of 0.060 inches for a larger droplet. The viscous material is pushed out of the syringe tip and contacts the substrate while it is still connected to the syringe tip. If the viscous material fails to contact the substrate, it will not adhere to the substrate and no droplet will result. The contacting of the viscous material with the substrate is called "wetting." After the viscous material contacts the surface of the substrate, the tip is pulled back and the resulting string is broken to form a droplet.
One problem with the prior art systems is the stringing or sticking of a bead of the viscous material to the nozzle. This can adversely affect the ability of the delivery system to dispense precise, quantitative amounts of liquid material. Stringing is most likely to occur at lower pressures, for instance, when the pressure in the syringe is ramping up or ramping down. For this reason, stringing also occurs more frequently as dispensing time decreases. Stringing of the liquid material from the nozzle tip during the final stage of dispensing may be avoided to some extent by making the internal pressure of the syringe negative. However, when dispensing again commences, a build-up of liquid at the nozzle tip almost invariably occurs, thus adversely affecting the stability of the subsequent extrusion. Also, to facilitate contact between the viscous material and the workpiece, a robot must constantly move the syringe toward and away from the workpiece, typically in up and down directions. This can significantly slow the manufacturing process.
Another approach to dispensing fluid from a syringe is disclosed in U.S. Pat No. 5,320,250. This dispensing apparatus includes a reservoir or syringe of a viscous material which communicates with a chamber that continuously receives the viscous material. A flexible resilient diaphragm forms an exterior wall of the chamber. An impact mechanism applies a predetermined momentum to the diaphragm to propel a predetermined, minute quantity of the viscous material from the chamber through a nozzle at a high velocity. This minute quantity takes the form of a very small jet of viscous material. As the impact energy is removed by means of a stop, the sudden decrease of the chamber pressure and the forward momentum of the jet "pinches" or stops the jet. For many viscous materials, the chamber is heated to control the viscosity of the material. The reservoir is preferably pressurized with gas to force the viscous material into the chamber. One problem with this type of design is that the high velocity imparted to form the jet of viscous material causes the jet tail to break into smaller droplets forming satellites.
Specific problems are encountered when dispensing solder pastes. Solder pastes typically comprise lead, tin or other metallic particles contained in a viscous material. One problem experienced with these pastes is that they tend to adhere to metallic parts of a dispenser. For example, adherence to metallic parts at the outlet, such as the outlet nozzle, can cause clogging problems over time. Also, when dispensing solder pastes in accordance with the descriptions set forth in the above incorporated applications, the constant impact of the valve member or valve shaft against the metal valve seat compacts the solder paste and causes it to flake, conglomerate and create clogging problems.
To overcome some of the problems of the prior art devices, a two-stage delivery system has been used where the viscous material resides in a syringe under a constant air pressure of about 4 psi to about 12 psi, depending on the viscosity. This insures steady flow of the material into a chamber of a rotary positive displacement pump. The pump dispenses as many as 25,000 dots of the viscous fluid per hour onto a high density, printed circuit (PC) board. Since the viscous material is pushed out of the syringe tip and contacts the substrate while it is still connected to the tip, however, the same problems exist as those described above relating to delivery from a syringe.
For at least these reasons, it would be desirable to provide a dispenser that can more rapidly and effectively apply minute amounts of viscous material to a substrate or workpiece.